Fungal prion
A fungal prion is a
Fungal prions have provided a model for the understanding of disease-forming mammalian prions. Study of fungal prions has led to a characterisation of the sequence features and mechanisms that enable prion domains to switch between functional and amyloid-forming states.
Sequence features
Prions are formed by portable, transmissible prion domains that are often enriched in asparagine, glutamine, tyrosine and glycine residues. When a reporter protein is fused with a prion domain, it forms a chimeric protein that demonstrates the conformational switching that is characteristic of prions. Meanwhile, removing this prion domain prevents prionogenesis. This suggests that these prion domains are, in fact, portable and are the sole initiator of prionogenesis. This supports the protein-only hypothesis.[citation needed]
A recent study of candidate prion domains in S. cerevisiae found several specific sequence features that were common to proteins showing aggregation and self-templating properties. For example, proteins that aggregated had candidate prion domains that were more highly enriched in asparagine, while non-aggregating domains where more highly enriched in glutamine and charged peptides. There was also evidence that the spacing of charged peptides that prevent amyloid formation, such as proline, is important in prionogenesis. This discovery of sequence specificity was a departure from previous work that had suggested that the only determining factor in prionogenesis was the overall distribution of peptides.[2]
HET-s prion of Podospora anserina
Podospora anserina is a filamentous fungus. Genetically compatible colonies of this fungus can merge and share cellular contents such as nutrients and cytoplasm. A natural system of protective "incompatibility" proteins exists to prevent promiscuous sharing between unrelated colonies. One such protein, called HET-s, adopts a prion-like form in order to function properly.[3][4] The prion form of HET-s spreads rapidly throughout the cellular network of a colony and can convert the non-prion form of the protein to a prion state after compatible colonies have merged.[5] However, when an incompatible colony tries to merge with a prion-containing colony, the prion causes the "invader" cells to die, ensuring that only related colonies obtain the benefit of sharing resources.
Prions of yeast
[PSI+] and [URE3]
In 1965, Brian Cox, a geneticist working with the
Further investigation found that [PSI+] is the result of a self-propagating misfolded form of
An important finding by Chernoff, in a collaboration between the Liebman and Lindquist laboratories, was that a
Laboratories commonly identify [PSI+] by growth of a strain auxotrophic for adenine on media lacking adenine, similar to that used by Cox et al. These strains cannot synthesize adenine due to a nonsense mutation in one of the enzymes involved in the biosynthetic pathway. When the strain is grown on yeast-extract/dextrose/peptone media (YPD), the blocked pathway results in buildup of a red-colored intermediate compound, which is exported from the cell due to its toxicity. Hence, color is an alternative method of identifying [PSI+] -- [PSI+] strains are white or pinkish in color, and [psi-] strains are red. A third method of identifying [PSI+] is by the presence of Sup35 in the pelleted fraction of cellular lysate.
When exposed to certain adverse conditions, in some genetic backgrounds [PSI+] cells actually fare better than their prion-free siblings;
[PIN+] / [RNQ+]
The term [PIN+] was coined by Liebman and colleagues from Psi-INducibility, to describe a genetic requirement for the formation of the [PSI+] prion.[18] They showed that [PIN+] was required for the induction of most variants of the [PSI+] prion. Later they identified [PIN+] as the prion form of the RNQ1 protein [19][20][21] The more precise name [RNQ+] is now sometimes used because other factors or prions can also have a Psi-inducing phenotype.[citation needed]
A non-prion function of Rnq1 has not been definitively characterized. Though reasons for this are poorly understood, it is suggested that [PIN+] aggregates may act as "seeds" for the polymerization of [PSI+] and other prions.[22][23][24] The basis of the [PIN+] prion is an amyloid form of Rnq1 arranged in in-register parallel beta sheets, like the amyloid form of Sup35.[25] Due to similar amyloid structures, the [PIN+] prion may facilitate the formation of [PSI+] through a templating mechanism.[citation needed]
Two modified versions of Sup35 have been created that can induce PSI+ in the absence of [PIN+] when overexpressed. One version was created by digestion of the gene with the restriction enzyme Bal2, which results in a protein consisting of only the M and N portions of Sup35.[26] The other is a fusion of Sup35NM with HPR, a human membrane receptor protein.[citation needed]
Epigenetics
Prions act as an alternative form of non-Mendelian, phenotypic inheritance due to their self-templating ability. This makes prions a metastable, dominant mechanism for inheritance that relies solely on the conformation of the protein. Many proteins containing prion domains play a role in gene expression or RNA binding, which is how an alternative conformation can give rise to phenotypic variation. For example, the [psi-] state of Sup35 in yeast is a translation termination factor. When Sup35 undergoes a conformational change to the [PSI+] prion state, it forms amyloid fibrils and is sequestered, leading to more frequent stop codon read-through and the development of novel phenotypes. With over 20 prion-like domains identified in yeast, this gives rise to the opportunity for a significant amount of variation from a single proteome. It has been posited that this increased variation gives a selectable advantage to a population of genetically homogeneous yeast.[27]
List of characterized prions
Protein | Natural Host | Normal Function | Prion State | Prion Phenotype | Year Identified |
---|---|---|---|---|---|
Ure2 | Saccharomyces cerevisiae | Nitrogen catabolite repressor | [URE3] | Growth on poor nitrogen sources | 1994 |
Sus35 | Saccharomyces cerevisiae | Translation termination factor | [PSI+] | Increased levels of nonsense suppression | 1994 |
HET-S | Podospora anserina | Regulates heterokaryon incompatibility | [Het-s] | Heterokaryon formation between incompatible strains | 1997 |
vacuolar protease B | Saccharomyces cerevisiae | death in stationary phase, failure in meiosis | [β] | failure to degrade cellular proteins under N starvation | 2003 |
MAP kinases | Podospora anserina | increased pigment, slow growth | [C] | 2006 | |
Rnq1p | Saccharomyces cerevisiae | Protein template factor | [RNQ+],[PIN+] | Promotes aggregation of other prions | 2000 |
Mca1* | Saccharomyces cerevisiae | Putative Yeast Caspase | [MCA+] | Unknown | 2008 |
Swi1 | Saccharomyces cerevisiae | Chromatin remodeling | [SWI+] | Poor growth on some carbon sources | 2008 |
Cyc8 | Saccharomyces cerevisiae | Transcriptional repressor | [OCT+] | Transcriptional derepression of multiple genes | 2009 |
Mot3 | Saccharomyces cerevisiae | Nuclear transcription factor | [MOT3+] | Transcriptional derepression of anaerobic genes | 2009 |
Pma1+Std1 [28] | Saccharomyces cerevisiae | Pma1 = major plasma membrane proton pump, Std1=minor pump | [GAR+] | Resistant to glucose-associated repression | 2009 |
Sfp1 [29] | Saccharomyces cerevisiae | Global transcriptional regulator | [ISP+] | Antisuppressor of certain sup35 mutations | 2010 |
Mod5 [30] | Saccharomyces cerevisiae | [MOD+] | 2012 |
[*The original paper that proposed Mca1 is a prion was retracted [31]]
See also
- Prion
- Sup35p
- Epigenetics
- Amyloid
- Proteopathy
- JUNQ and IPOD
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